Nanowire arrays are seeing increasing use in a variety of applications. See, e.g., U.S. Patent Application Publication No. 2009/0256134, “Process for Fabricating Nanowire Arrays,” filed Apr. 14, 2009. An exemplary silicon nanowire array may include a collection of silicon nanowires, on the order of 100 nm in diameter, on the order of several micrometers in height, and of approximately cylindrical or frustoconical shape. The axes of the nanowires run approximately parallel to each other. Each is attached at an end to a silicon substrate and is very roughly perpendicular to that substrate.
A process for fabricating nanowire arrays is described in U.S. Published Patent Application No. 2009/0256134. In this process, one deposits nanoparticles and a metal film onto the substrate in such a way that the metal is present and touches silicon where etching is desired, and is blocked from touching silicon or not present elsewhere. One submerges the metallized substrate into an etchant aqueous solution comprising hydrofluoric acid (HF) and an oxidizing agent. In this way, arrays of nanowires with controlled diameter and length are produced.
Relevant information regarding silicon fabrication processes known to those of skill in the art can be found, for example, in Sami Franssila, Introduction to Microfabrication (John Wiley & Sons 2004), and the references cited therein.
A silicon nanowire array can reduce the reflectivity of a solar cell surface. Other types of nanostructuring can also achieve this effect.
A silicon nanowire array on top of a silicon substrate, can alter the opto-electrical properties of the bulk silicon substrate. For example, a silicon nanowire array reduces the reflection of the silicon substrate, reduces the reflection at off-angles of incidence, and increases the absorption of the silicon in ways similar to traditional pyramids or light trapping mechanisms used in solar cells.
Some of the altered optical-electrical properties of silicon nanowires compared to bulk silicon are beneficial for solar cells. However, in order to form a solar cell, the two sides of a p-n junction need to be connected to the outside world. Unfortunately, contacting nanowires is not always easy.
One device design for nanowire solar cells places vertically aligned nanowires on top of a bulk (non-nanostructured) substrate. In this design, the back contact can be made from the backside of the substrate. The front contact, however, is more difficult to make.
For the types of solar cells currently manufactured, not using nanowire arrays, it is common to make contacts by screen printing. Screen printing is robust, has a high throughput, and is low-cost. The front and back contacts of a solar cell are typically formed in separate steps. For most cell designs, silver is applied to the front, and aluminum to the back. For the front, paste is squeezed through a stainless steel or polyester fine metal mesh screen with an adjustable and finely controlled force delivered through metal or polymer squeegee. The screen defines a comb-like (finger line array and crossed bus bars) pattern designed to provide sufficient conductivity while minimizing optical shading from the metal lines. The paste is then dried at temperatures of 100-200 C to drive off organic solvents and fired at around 800 C to diffuse in the metal to establish a low contact resistance junction. For the back, an aluminum based paste is screen printed on the rear surface, establishing electrical contact and functioning as a back surface field. The aluminum is applied as a paste squeezed through a fine mesh screen, then fired at high temperatures to drive off organic solvents and diffuse in the aluminum to establish a low contact resistance junction. Although a continuous contact will result in lower resistance, commercial wafers utilize a back contact with an embedded mesh structure to reduce paste usage and minimize wafer warping during the subsequent high temperature processing steps. The pattern is defined in the screen by photolithography, although laser cut metal stencils may be utilized for smaller line widths. Automatic screen printers are available that are capable of in-line, continuous operation with high throughput. These machines accept wafers from packs, cassettes or a belt line, place them with sufficient accuracy under the screen and deliver the printed wafers to the belt line. Detailed methods for screen printing are described in reference (1).
One of ordinary skill in the art would expect that screen printing on nanowires would be difficult. For one, the nanowires may break or bend when the squeegee is moved across the surface of the cell to remove excess screen printing paste. In addition, one of ordinary skill in the art may not expect the nanowires to survive the high temperature fire required to drive off the organic materials in the pastes. Although the present assignee and others have contacted nanowire arrays with other processes such as electrochemical deposition (6) and sputtering, screen printing is the dominate process in solar cell manufacturing and is cost-effective. Hence developing a nanowire array that can be contacted via screen printing is of commercial importance.